In the manufacture of printing paper, pigmented coating compositions are applied by, for example, blade, bar, air-knife or reverse-roll type coating methods, usually at high speeds. However, the said coating methods are non-contoured (with the exception of air-knife coating method) onto rough substrates which means that any irregular substrate surface will lead to non-uniform coating thickness, which may result in irregularities during the printing process.
Curtain coating methods are now being developed in the paper industry for coating paper webs, to achieve uniformity in surface application which is a necessary condition for obtaining a good final print rendering.
Curtain coating processes are well known and widely used for the application of one or more liquid layers onto the surface of a moving support in the photographic industry. Indeed, this technology was developed for photographic films that require the deposit of many different coats, usually between 8 and 10, with severe constraints on the surface condition and also the thickness of applied coats.
Curtain coating is a pre-metered coating process which means that only the required amount of coating liquid needed on the web is pumped through the coating head. The curtain coating process is based on free flow on a surface from a coating head located above the surface to be coated. The support is coated by forming a freely-falling vertical curtain of liquid so that it impinges onto the support. A controlled relationship is maintained between the flow rate of the liquid and the speed at which the support is moved so that the curtain is stable and has a uniform flow rate across its width to obtain a layer of the coating onto the substrate. The coating head is defined using properties of the coating fluid, so as to obtain the most uniform possible coating film, thickness in the running direction or the transverse direction of the machine.
One of the advantages of curtain coating is the superior quality and more uniform surface of the substrates, namely paper webs, that can be attained.
Another advantage is the lack of contact between the coating head and the support, unlike contact coating processes such as blade and rod coating. This provides a means of eliminating forces applied on the support during coating, which causes web breakage in particular, and can have a non-negligible effect on the increase in the machine speed, and consequently can reduce production costs.
Another advantage of curtain coating is the possibility of applying two or more coats simultaneously.
Despite many attempts to coat substrate webs with one or more coating layers using curtain coating processes, the main limiting factors to curtain coating remain the curtain stability and the air entrainment, which gives rise to the inclusion of air between the coating composition and the web leading to numerous bubbles and irregularities in the coating. Puddling of the curtain can also be an issue and lead to the formation of a heel at the curtain impingement zone, usually as a result of the coating fluid being too low in viscosity. This phenomenon can lead to coating non-uniformities, and can also induce air entrainment at relatively low web speeds. Highly viscous and elastic curtains tend to ‘pull’ along the web, which can also lead to a non-uniform coating due to fluctuations at the curtain impingementzone. In curtain coating, uniform layer(s) are only obtained if the operational variables are held within fairly precise limits. These limits define the coating ‘window’. One example is air entrainment, as described in U.S. Pat. No. 5,391,401 (Blake et al.). This patent describes a method to alleviate the issues of air entrainment. Therein is described an optimum relationship between viscosity and shear rate for curtain coating. The desired rheological profile promotes a low viscosity at the shear rates expected near the dynamic wetting line, where the coating wets the substrate, and a high viscosity at the much lower shear rates expected in the other regions of the flow. Coating composition can be increased in viscosity by the addition of thickening agents that interact with the binder, which has the effect of increasing the mix viscosity at low shear rate without substantially raising its viscosity at high shear rate, implying that a high viscosity at high shear rate is a disadvantage. Much of this work has been recorded for formulation containing binder, especially gelatine, along with silver halide grains for use in photographic applications. U.S. Pat. No. 5,393,571 (Suga et al.) describes the alleviation of air entrainment and puddling of the curtain coating by using a mix of viscosity superior to 90 mPa·s at low shear rate (10 s−1) onto a rough surface (0.3 μm). U.S. Pat. No. 6,099,913 (Clarke et al.) describes the formation of a coated layer adjacent to the substrate surface having a viscosity of 90 mPa·s to 220 mPa·s at a shear rate of 10000 s−1, which can form a free-falling curtain and allows for higher coating speeds to be attained without air entrainment. These applications are apparently suitable for photographic formulations generally containing gelatine and silver halide grains, which typically have a low solids content.
A second example of a limiting factor is curtain stability which is related to the ratio of inertial to surface tension forces (Weber number). This implies that higher flow rates and lower surface tensions are beneficial to curtain stability. However, in certain cases high flow rates are undesirable especially when high solid content mixes are used and lower coat weights are required.
Many typical paper coating compositions are highly pigmented and of high solids content and inherently are shear-thinning (pseudo-plastic) in nature. This type of rheology is useful for applications with blade, bar, reverse roll, slot or slide coating techniques. However, it has been observed that numerous pigmented paper coating formulations do not form a stable curtain at low flow rates (especially when Q, the flow rate per unit die length, is equal to or less than 10−4 m3/(s·m) for aqueous systems).
If the curtain coating method is to be used to coat high solid content paper formulations at low coat weights, this can only be achieved currently by utilising faster web speeds. However, at faster web speeds air entrainment becomes a real issue especially when coating onto smooth, less porous substrates.
Diluting the mix in order to run at slower web speeds is not an option. High solids mixes are preferred in the coating process as there is less demand on the drying capacity and it allows for lower grammage raw base (<80 g/m2) to be used, which can readily break during the process under high wet coating weights. Higher solid content mixes impart improved properties on the coated media, for example, higher gloss. Diluting mixes also lowers mix viscosity, which can lead to heel formation at the curtain impingement zone, if it becomes too low.